Method for producing orthoferrite single crystals

ABSTRACT

ORTHOFERRITE SINGLE CRYSTALS ARE PRODUCED BY FIRST PRODUCING AN ORTHOFERRITE POLYCRYSTALLINE BODY HAVING AN FE2O3 CONTENT RANGING FROM ABOUT 49.8 MOL PERCENT TO 50.4 MOL PERCENT AND THEN SUBJECTING SAID BODY TO A FLOATING-ZONE METHOD IN OXYGEN AT A PRESSURE RANGING FROM 1 TO 10 ATOMSPHERES.

United States Patent O1 3,748,271 Patented July 24, 1973 hoe 6 Claims ABSTRACT OF THE DISCLOSURE Orthoferrite single crystals are produced by first producing an orthoferrite polycrystalline body having an Fe O content ranging from about 49.8 mol percent to 50.4 mol percent and then subjecting said body to a floating-zone method in oxygen at a pressure ranging from 1 to 10 atmospheres.

The present invention relates to a method of preparing orthoferrite single crystals by the floating-zone method.

The chemical formula of orthoferrite is RFeO where in R denotes an element selected from the group consisting of yttrium and rare earth elements. Complex orthoferrite, such as of the formula R R FeO wherein R and R both denote yttrium or one of rare earth elements and which is obtainable by replacing a part x of R in RFeO with another element R, is meant to be included in the trem orthoferrite. Recent uses of orthoferrite single crystals have been in cylindrical domain device applications.

Various methods, such as the flux method and hydrothermal synthesis, are known for producing orthoferrite single crystals. However, With such methods, the growth of single crystals is too slow and large single crystals are not obtainable. It would be desirable to provide a floatingzone method for producing large single orthoferrite crystals.

It is known that single crystals may be produced by the floating-zone method, providing that the object compound crystallizes as a solid phase congruently. This requirement is satisfied by orthoferrites of stoichiometric composition, that is, consisting of 50 mol percent Fe O and 50 mol percent R Where the compositions of orthoferrites are slightly different from the stoichiometric composition and tend towards an excess of the iron component or an excess of the rare earth component, the floating-zone method is hardly useful in the manufacture of single crystals thereof. Since marked or significant changes in magnetic do main structure and other magnetic properties of the single crystal do not readily occur, it is possible to employ single crystals of such compositions in practical applications. Generally, when the composition varies from the stoichiometric composition, gas bubbles develop in or derive from the molten zone, as will be discussed later, such that a stable molten zone is not formed. This interrupts the stable growth of single crystals. In addition, numerous spherical pores remain in the single crystal grown under such conditions. Thus, a homogeneous single crystal is not obtainable with such compositions.

Because of the foregoing problems, it is generally required that the polycrystalline raw material employed for producing a single crystal be stoichiometric in composition. However, this has economic disadvantages in commerce insofar as yield and productive efficiency are concerned.

Generally speaking, iron oxides and oxides of yttrium and other rare earth metals (including the metal salts and metal hydroxides which are capable of being converted into oxides at higher temperatures) used as starting materials for making orthoferrites are usually hygroscopic and difiicult to control with regard to the metallic contents thereof. Moreover, it is difficult to regulate amounts of iron picked up in ball mills and other impurities which are absorbed in the process of preparing a polycrystalline raw material. Therefore, it is very difiicult to prepare polycrystalline raw material of homogeneous composition in which the molar ratio of Fe O to R 0 has the desired stoichiometry. The actual composition usually falls within the range of 50:05 mol percent of Fe O When a single crystal is prepared according to the floating-zone method from polycrystalline raw material whose composition varies or deviates to some extent from the stoichiometric composition, then bubbles are formed depending upon the amount of deviation from the stoichiometric composition within the molten zone, the bubbles thereafter coagulating and gradually growing into large bubbles in the molten zone. Finally, they escape from the molten zone and vanish. When this process is repeated in the course of the crystal growth, the molten zone which should otherwise be stable undergoes shock and, in extreme cases, the molten zone is broken.

It is therefore an object of the present invention to provide a method of making homogeneous orthoferrite single crystals by the floating-zone technique at an improved yield rate, even where there are slight differences in compositions among polycrystalline raw materials and where the compositions of the raw materials deviate from stoichiometry.

According to the invent-ion, crystal growth by the floating-zone method using polycrystalline raw material is carried out in an oxygen-containing atmosphere, e.g. air or in an atmosphere of oxygen, at one to ten atmospheric pressure, depending upon the deviation of the composition of the polycrystalline raw material from the stoichiometric relationship of the compound of orthoferrite.

Optimum atmosphere is determined by the composition of the polycrystalline raw material. Where the raw material contains just 50.0 mol percent Fe O oxygen of at least one atmospheric pressure v(atm.) can be used. Where the Fe O content of the raw material deviates from 50.0 mol percent by x mol percent, oxygen having (20]x]1) or more atmospheric pressure should be used as an atmosphere during crystal growth by the floatingzone method using the raw material. However, the pressure should not exceed 10 atmospheres as oxygen pressure more than 10 atmospheres often produces an abnormal projection in part of polycrystalline raw material in contact wtih the molten zone and makes the molten zone unstable. Therefore pressures above 10 atmospheres should be avoided. Furthermore, if the deviation of the composition of polycrystalline raw material is so large that the Fe O content thereof lies outside the range of 49.8 mol percent to 50.4 mol percent, then a different phase other than orthoferrite may tend to deposit. Therefore, compositions outside the foregoing range should be avoided.

According to the invention, by utilizing a single crystal growth furnace for the floating-zone method in which the atmosphere is optionally controllable, the yield rate in the manufacture of such crystals is markedly improved.

As illustrative of the invention, the following preferred embodiments are given.

EMBODIMENT 1 Feed rods of polycrystalline raw materials of YFeO were prepared by the usual method of making ferrite ceramics. A feed rod was placed in a floating-zone apparatus. A molten zone was formed between the feed rod and a seed crystal by heating to a temperature above the melting point (about 17-00" C.) of YFeO and the zone moved upward at a constant rate. The feed rod and the seed crystal were rotated at 30 rpm. in the opposite direction in order to produce a good stirring action and reduce temperature gradients. The direction of growth was generally parallel to the c-axis.

Experiments on crystal growth of YFeOg were carried out in an oxygen atmosphere having oxygen pressures of l to 10 atm. and at a growth rate of about 10 mm./hr. by using feed rods having compositions of 49.5 to 50.7 mol percent Fc O Experimental results particularly relating to gas bubble derivation from the molten zone and other phase precipitation in the crystals are shown in Table 1.

TABLE 1 Composition 1 Gas Other FezOa YzOa Oxygen bubble phase Sample (mol per- (mol perpressure derivaprecipinumber cent) cent) (atrm) tion I tattoo 2 49. 5 50. 5 1 Yes Yes. 49. 5 50. 5 10 49. 8 50. 2 1 49. 8 50. 2 3 50. l 49. 9 1 50. 1 49. 9 10 50. 3 49. 7 1 50. 3 49. 7 4 50. 3 49. 7 6 50. 3 49. 7 10 50. 4 49. 6 5 50. 4 49. 6 7 50. 7 49. 3 1 50. 7 49. 3 10 1 Composition is that of feed rod before sintering and determined by the chemical analysis.

1 Yes denotes that the phenomena were more or less observed and N denotes not observed.

As will be seen from Table 1, gas bubble derivation decreases with the decrease of the deviation of feed rod composition from stoichiometry and with the increase of oxygen pressure. In the case of nearly stoichiometric composition (Sample Nos. and 6), no gas bubble derivation was observed, whether under 1 oxygen atmospheric pressure or under a higher oxygen pressure of ten atmospheres whereby smooth crystal growth was obtained. Where the YFeO feed rod has an excess Fe O content by about 0.3 mol percent above stoichiometry (Sample Nos. 7 to gas derivatives from the molten zone under 1 atmosphere, while gas derivation does not occur under a higher oxygen pressure of 6 atm. or more. Where the deviation increases up to 0.4 mol percent Fe O excess (Sample phases can be produced it x satisfies the relationship 0.2x0.4. In this case, if ]x[0.l, no gas bubble is derived from the molten zone under 1 atm. oxygen pressure. If |xi 0.l, no gas bubble derivation occurs under an oxygen pressure of (20|x[-l) atmospheres or more.

Although the growth rate was 1 0 mm./hr. in the above experiments, results did not vary under a growth rate of 3 mm./hr. to 20 mm./hr.

EMBODIMENT 2 By using the same technique as Embodiment 1, single crystals of TbFeO ErFeO and YbFeO were grown. Typical results are shown in Table 2. High oxygen pressure of 20lxl-l or more atmospheres is also effective in these cases, as is seen from Table 2.

TABLE 2 Composition 1 Gas Other Oxygen bubble phase F9203 (mol R203 (mol pressure derivaprecipipercent) percent) (atrn.) tion tation I 50.2 Tb2Oa,49.8 1 Yes--- No. 50. 2 T132014, 49. 8 7 N0 No. 50. 3 EI203,49. 7 1 Yes No. 50. 3 EH03, 49. 7 8 No No. 49.9 YbzOa,50.1 1 No No. 50.3 YbzOa, 49.7 1 Yes No. 50. 3 Ybz03, 49. 7 7 o No.

NoTn.See footnotes 1 and 2 at end of Table 1.

EMBODIMENT 3 Sample Nos. 22 to 24 of Table 3 show that the invention is also effective for YFeO in which Fe is partially replaced with Co. Since Co occupies the site of Fe in the crystal lattice, the stoichiometric composition of the raw material in this case is mol percent 0 and 50 mol percent in total of Fe O plus C0 0 Co may replace up to 5 atomic percent of Fe. Sample No. 25 of Table 3 represent that the invention can be successfully applied to production of YFeO in which part of Fe is replaced with Co and Ti. Since Co and Ti occupy the sites of Fe ions in the crystal, the stoichiometric composition of the raw material is 50 mol percent 0 and 50 mol percent in total of F6203, C00 and TiO in this case. Co and Ti may replace up to 5 atomic percent of Fe. Where the feed rod composition Fe O '+Co O or Fe O +CoO+TiO is 50+x mol percent, the floating-zone melting under an oxygen pressure of 20114-1 atm. or more can produce good single crystals.

m gnum, not, 50.0

N 0'rE.-See footnotes l and 2 at end of table 1.

Nos. 11 and 12), still high oxygen pressure of 7 atmospheres or more is necessary to stop gas bubble derivation. If the feed rod composition deviates toward the Y O excess side by 0.2 mol percent from the stoichiometry (Sample Nos. 3 and 4), a higher oxygen pressure of 3 or more is needed. On the other hand, application of oxygen pressure cannot stop gas bubble derivation and precipitation of other phases than orthoferrite in the resultant single crystal, if the deviation of the composition is too large (Sample Nos. 1, 2, 13 and 14).

The deviation of feed rod composition from the stoichiometry and the oxygen pressure necessary to stop the gas bubble derivation appears to have nearly a linear relationship. Summarizing the foregoing, Where the Fe 0 composition of polycrystalline raw material is expressed by 50+x mol percent, single crystals having no other Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

What is claimed is:

1. A method of producing orthoferrite single crystals comprising the steps of preparing an orthoferrite polycrystalline body consisting essentially of Fe O and one oxide selected from the group of Y O T b 0 Er O and Yb O the Fe O content being 50+x mol percent with x ranging from 0.2 to +0.4, and subjecting said polycrystalline body to floating zone melting by heating said polycrystalline body at a temperature higher than the melting point of said polycrystalline body to form a molten zone between said polycrystalline body and a seed crystal and moving said molten zone at a rate of 3 to 20 mm. per hour in an oxygen atmosphere having a pressure of at least one atmospheric pressure where lxl 0.1 and at least 20]x[l atmospheric pressure where [x[ 0.1, the pressure of said atmosphere being not more than ten atmospheric pressure.

2. The method claimed in claim 1, in which the oxide is Y203.

3. A method of producing orthoferrite single crystals comprising the steps of preparing an orthoferrite polycrystalline body consisting essentially of Fe O and one oxide selected from the group of Y O Tb O Er O and YbgOg, wherein an amount ranging up to 5 atomic percent of Fe is replaced by Co as C0 0 and wherein the Fe O and the C0 0 content is 50,+x mol percent with x ranging from O.2 to +0.4, and subjecting said polycrystalline body to floating zone melting by heating said polycrystalline body at a temperature higher than the melting point of said polycrystalline body to form a molten zone between said polycrystalline body and a seed crystal and moving said molten zone at a rate of 3 to 20 mm. per hour in an oxygen atmosphere having a pressure of at least one atmospheric pressure where |xl0.1 and at least 20]x[l atmospheric pressure where ]x[ 0.1, the pressure of said atmosphere being not more than ten atmospheric pressure.

4. The method claimed in claim 3, in which the oxide iS Y203.

5. A method of producing orthoferrite single crystals comprising the steps of preparing an orthoferrite polycrystalline body consisting essentially of Fe O and one oxide selected from the group of Y O T-b O Er 0 and Yb O wherein an amount ranging up to 5 atomic percent of Fe is replaced by a combination of Co and Ti as C00 and TiO and wherein the Fe O the C00 and the TiO content is +x mol percent with x ranging from 0.2 to {+0.4, and subjecting said polycrystalline body to floating zone melting by heating said polycrystalline body at a temperature higher than the melting points of said polycrystalline body to form a molten zone between said polycrystalline body and a seed crystal and moving said molten zone at a rate of 3 to 20 mm. per hour in an oxygen atmosphere having a pressure of at least one atmospheric pressure Where |x[0.1 and at least 20 ]x]l atmospheric pressure where |x| 0.l, the pressure of said atmosphere being not more than ten atmospheric pressure.

6. The method claimed in claim 5, in which the oxide is Y203.

References Cited UNITED STATES PATENTS 3,272,591 9/1966 Rudness et al. 252-6257 X 3,414,372 12/1968 Paulus et al 23-51 R 3,485,759 12/1969 Kolb et al. 252-6257 OTHER REFERENCES Plaskett et al., IBM Technical Disclosure Bulletin, vol. 13, No. 9, p. 2632, February 1971.

OSCAR R. VERTIZ, Primary Examiner J. COOPER, Assistant Examiner US. Cl. X.R. 

